Apparatus for testing electric coils



May 27, E947. F. J. WATSON Erm,

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mmsrmmf Patented May 27, 1947 Frederick James Watson, Old Windsor, and Felix Moutran, Slough, England, assignors to Self- Priming Pump & Engineering Co. Limited, London, England, a British company Application June 17, 1943, Serial No. 491,188 In Great Britain May 6, 1942 This application corresponds to the application of Self-priming Pump 8: Engineering Co. Limited, Frederick James Watson and Felix Moutran, Serial No. 6106/42, which was illed in Great Britain on May 6, 1942.

This invention consists of improvements in or relating to apparatus for testing electric coils and z claims. '(ci. ras-183) the apparatus may embody, for each of the two tests (resistance and number of turns) two circuits appropriate to the testing of coils of two different voltage standards, say 12-volts and 24- Ivolts, and switch means for selectively determining which voltage circuits shall be operated.

particularly it relates to a device for checking the resistance and the number of turns in electric coils like the eld coils of a dynamo electric machine.

In normal times the measurement of the resistance of a coil (such as a eld coil) can' be carried out on a Wheatstone bridge type of apparatus with great exactitude. Electric inductanoe measurements in coils can likewise Ibe carried out with accuracy but in turning out smallelectric motors or dynamos or coils for such machines on a system of mass production, it has become necessary to provide va rapid check on` the The unitary structure may comprise a box for housing the various parts of the testing equipment, an electromagnetic core frame for receiving the coil under test being accommodated in a portion of the box provided with an opening (for insertion'of the coil) furnished with a hinged coverplate, and switches automatically controlled by resistance of a coil and on the number of turns inthe coil simply to ensure that the coil in question comes up to the prescribed standard in both these respects and the work may have to be done by unskilled labour. y

According to this invention the apparatus is a unitary structure comprising a mounting for the coll to be tested, a Wheatstone bridge or equivalent device for checking the ohmic resistance of the coil by including it as a branch of the bridge, means for checking the number of turns in the coil by an A. C. circuit including a transformer whereof the secondary may be constituted wholly or in part by the coil under test and includes in circuit a current-indicating instrument, and means for selectively coupling the coil under test in the Wheatstone bridge circuit or in the secondary circuit of the transformer.

According to a feature of the invention both the resistance and turns tests may be carried out using vthe same galvanometer or equivalent current-indicating instrument, means being provided for coupling the galvanometer selectively either in the Wheatstone bridge system or through a rectier in the secondary coil circuit of the transformer. The galvanometer may have a screen mounted in front of a pointer and formed with two slots through which different portions in the length of the pointer may be observed, the two slots being appropriate respectively to the resistance and turns tests andthe location and length of each'slot being such that its opposite ends define prescribed plus and minus variations from a standard.

According to a further feature of the invention the cover-plate being provided for closing the testing circuits. The necessary contacts for con#I necting the windings of the coil under test with the testing circuits may also be controlled by the lclosing of the cover-plate. Preferably the test- 'ing circuits include coloured lights to indicate which of the two tests is in progress.

One apparatus embodying the foregoing and other features oi the invention will now be 'described with reference to the accompanying drawings, in which- Figures 1, 2 and 3 show the apparatus infront elevation, side elevation and plan respectively, and

Figure 4 is a wiring diagram.

The apparatus is designed to test motor field coils for both 12- and 24-volt units, the test in each case being carried out in two stages, iirstly the test of the ohmic resistance of the coil and secondly the check on the number of turns in the coil.

The general layout of the apparatus (the circuit control of which will later be described) is, as follows. equipment are accommodated in a box 30 having a sloping front wall 3l, behind the upper part of which resistance units 5, 6 and a double resistance unit 2, 3 constituting three branches of a Wheatstone bridge are mounted. The Wheatstone bridge is energised from a 2li-volt D. C. supply 40. A galvanometer 8 is mounted behind the sloping front wall 3l below the resistance and is adapted .(in a manner presently to be described) to be included selectively either in the resistance test or in the turns test. The pointer of the galvanometer is movable behind an observation plate 32 (Figure 1) in which two inspection slots 33, 34 appropriate respectively to` the resistance and number 'of turns tests, are provided. 'I'he lower end of the sloping wall iii of the box leads to a front horizontal compartnient 35 (Figure 2) in which is located the socket,

The various parts of the testing in the form of an electromagnetic core frame 3B. to receive the coil 23 to be tested. A single-phase transformer I I is mounted at the back of the box on the base thereof. The primary of the transformer II is supplied from a 24d-volt A. C. source 4I, and the secondary of this transformer is connected through a resistance I (mounted behind the front wall 3l of the instrument box alongside the bridge rcsistances 2, 3. 6, 6) to a coil 22 in the core of the receiving unit 38. The front compartment 35 is furnished with a hinged coverplate 31 pivotally mounted on a spindle 38 and urged by springs 38 towards the closure position. The cover-plate 31 controls contacts 2|, 24, 25 for the iield coil under test, the contacts 2 I, 25 taking in both shunt and series windings of the coil and contacts 2 I, 24 taking in the shunt winding only. The cover-plate 31 also controls automatic switches I9, 20, 28 for the various test circuits. The switches I9, 28 are included respectively in the turns and resistance test circuits. The switch 26 operates a green lamp I off the 24D. C. supply 40. The illumination of the lamp 1 is visible through an observation slot 42 in the front wall 3I or the instrument box and is designed to show that the cover-plate 31 is well pressed home before any readings are made.

Manually operable switches I2, I4, I5, IB, II are mounted in the box 30 between the electromagnetic core frame 36 and the transformer II. The switch I2 is operable by an external knob I3 and determines the selection .of the appropriate circuits for the testing of 12-volt or. alternatively, 24-volt field coils. The switches I4, I5, I8 and I1, which are simultaneously operable by an external knob I8, control the selection of various circuits appropriate respectively t the resistance and number of turns tests.

A white lamp 9 and a blue lamp I8, operated oil the 24-volt D. C. supply, are arranged to indicate. through an observation window A43 in the front wall 3I of the instrument box, whether the resistance or number of turns test is in progress. A lamp 4 operated oi the 24o-A. C. supply provides the necessary illumination at the slots 33, 34 in the galvanometer dial plate 32.

As already indicated, the apparatus in the present example is designed to test motor ileld coils for 12- and 24-volt units. The normal total value of resistance of one such eid coil (shunt and series windings) if 9.70 ohms for the 12-volt unit and 30.77 ohms for the 24-volt unit. In carrying out the resistance test the field coil 23 to be tested is arranged to constitute one of the four branches of the Whcatstone bridge, the remaining three branches being constituted by the resistances 5, 8 each of 120.8 ohms, and the double unit resistance 2. 3, the unit 2 having a resistance of 21.07 ohms and the unit 3 a resistance of 9.7 ohms. The arrangement (which will later be described in more detail) is such that when a 12-volt coil is being tested the unit 2 of 21.07 ohms is short-circuitcd leaving in circuit only the unit 3 of 9.7 ohms resistance, and when a 24- volt field coil is under test both units 2. 3 are '.n

circuit giving a total resistance of 2l.07-I-9.7 ohms.

" fi. e. 30.77 ohms.

and 29.232 ohms. The galvanometer is so arranged that its zero reading corresponds to the mid-position in the length of the arcuate slot 33 in the dial plate 32 and the length of the slot is such that its ends correspond to the limit values indicated above. Thus if on test the pointer is beyond the slot, either to the left or right, this indicates that the ohmic resistance oi' the field coil under test is higher or lower than the specined 5% and should be rejected.

In the present example the primary coil 22 in the electromagnetic core comprises forty turns of 23 S. W. G. and the arrangement is such that when the number or turns of the secondary 23 constituted by the coil under test are correct-in the present case turns for the 12-volt coil and 300 turns for the 24-volt coil-the pointer of the galvanometer will show at the mid-position of the lower slot 34 in the'dial plate 32. The length of the slot 34 is such that its ends define positions corresponding to variations of 15% turns above or belowthe correct values. Thus if the number of turns of a coil under test is over 15% or under 5% the pointer will disappear to the left or to the right respectively of the slot 34 and the coil under test should then be rejected.

The operation of the switchesl I2 and I4 to I1 will now be described with reference to the wiring diagram (Figure 4).

In the upper position of the switch I2' the lefthand blade of the switch short-circuits the 21.07 ohm resistance unit 2, putting in circuit the 9.7 ohm resistance unit 3 corresponding to the 12- volt iield coil. In the lower position o1 the lefthand blade of the switch I2 the 21.07 ohm resistance unit 2 is included in the circuit and the total resistance becomes 30.77 ohms corresponding to the 24volt test requirement. The righthand blade of the switch I2` is utilised for selecting the appropriate circuit in the number of turns test corresponding to a 12-volt or, alternatively, 24vo1t neld coil. In its upper position the right-hand blade of the switch I2 short-circuits a portion of the primary coil 22 and of the resistance I included in the primary coil circuit. In the lower position of the right-hand blade of the switch I2 the whole oi.' the primary coil 22 and oi' the resistance I is included in the testing circuit.

The switch I4 is arranged to control the galvanometer circuit appropriate to the resistance and turns tests respectively. In its upper posisition the galvanometer 8 is connected to the Wheatstone bridge (resistance test) and in its lower position is connected in series with a resistance 21 and a rectifier 28 (turns test). The purpose of the rectiiier 28 is to make use of the same galvanometer on A. C. circuit.

The switch III is arranged to operate similarly to switch I4, the upper position of the switch blades being appropriate to the resistance test and the lower position appropriate to the turns test.

Similarly, the switch I6 in its upper and lower positions gives appropriate connections for the resistance and turns tests respectively. The lefthand blade, in its upper position, takes in the whole field coil under test (shunt and series windings) across the coil contacts 2|, 25, while in its lower position connects, across the coil contacts 2|, 24, the shunt winding only in circuit for the turns test. The right-hand blade of the switch I6 in its upper position takes into circuit the white lamp 3 showing that the resistance test is-in progress, and in its lower position the right-hand blade takes into circuit the blue lamp I indi-'- cating that the turns test is in progress.

The switch I1 in its upper position connects the 24-volt D. C. supply to the Wheatstone bridge and in its lower position connects the 24U-volt A. C. supply to the primary of the single-phase transformer II.

In making a test, the coil to. be tested is -ilrst inserted in the core frame 36 and the lid 31 of th'e iront compartment 35 is securely closed down. As already indicated, the closing of the lid automatically establishes the coil contacts 2l, 24 and 25 and also the circuit contacts I9, 20 and 25. The operator then turns the knob I3, controlling the dual voltage range switch I2, in a direction appropriate to the voltage of the coil to be tested. The operator then turns th'e knob I8 in a direction appropriate' to the resistance test, all the switches I4, I5, I6. and I'I being simultaneously thrown into positions appropriate to the resistance test circuits. After the galvanometer reading has been taken the knob I8 is turned in the opposite direction and the switches I4 to I1 kare thereby thrown simultaneously into positions appropriate to the circuits of the turns test and the galvanometer reading is again taken.

We claim:

1. Apparatus for testing electrical coils as to ohmic resistance and number of turns in the coil, saidapparatus comprising a, unitary structure which includes, in combination, a single current- I indicating instrument, a mounting for receiving the coil under test including contacts for the winding terminals thereof, a Wheatstone bridge supplied by a D. C. circuit, a transformer pri mary associated. with said -mounting and supplied by an A. C. circuit, switching means for electrically connecting said contacts for the coil,

there is provided a unitary device co `bining resistance and reactance testsfior test coils, the single current-indicating instrument being provided with a pointer,v a cover for said instrument and pointer, two slots formed' in said cover through which two different portions of the length of the 'pointer may be observed during certain portions of its movement, the slots being appropriate respectively to the reading of indications oi the resistance and number of turns tests. and the location and lengths of each slot being such that its opposite ends deilne prescribed plus and minus variations from a standard.

ough said rectier, in the secondary circuit of said transformer whereby 2. Apparatus for testing electrical coils as to ohmic resistance and number of turns in the coil, said apparatus comprising a unitary structure which includes, in combination, a single currentindicating instrument, a mounting for receiving the coil under test including contacts for the winding terminals th'ereof, a Wheatstone bridge supplied by a D. C. circuit, a transformer primary associated with said mounting and supplied by an A. C. circuit, switching means for electrically connecting said contacts for the coil under test selectively either as a branch of the Wheatstone bridge or as part at least of the secondary of .a transformer, the said. primary of which is associated with the mounting, a rectifier, switch means for electrically connecting said instrument selectively eith'er in the Wheatstone bridge system, or through said rectifier; in the secondary circuit of said transformer whereby there is provided a unitary device combining resistance and reactance tests for test coils, the single current-indicating instrument being provided with a pointer, a cover for said instrument and pointer. two slots formed in said cover through which two different portions of the length of the pointer may be observed during certain portions of its movement, 'the slots being appropiate respectively to the reading of indications of the resistance and number of turns tests, and th'e location and, lengths of each slot being such -that its opposite'ends denne prescribed plus and minus variations from a standard, and dierently colored lights being disposed in the two respective circuits to indicate which of the REFERENCES GITED vr'Tle""'fillowing references are of record in the lleof `thispatent:

UNITED s'rA'rEs PATENTS Name Date Conant Oct. 15, 1901` Carrington Jan. 14, 1930 Paulson Apr. 11, 1939 N umbcr Cooke May 24, 1927 Butler Apr. 1, 1930 Wickson Jan. 2, 1934 Banker Dec. 12, 1944 Smith Bent. 8, 1942 Cullin Jan. 1, 1924 Macndle May 22, 1934 Inwensteln May 3, 1938' Stearns Dem/8, 1942 Pickard Nov. 17, 1925 

